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''Echinococcus granulosus,'' also called the Hydatid worm or Hyper Tape-worm or Dog Tapeworm, is a cyclophyllid cestode that parasitizes the small intestine of canids as an adult, but which has important intermediate hosts such as livestock and humans, where it causes cystic echinococcosis, also known as hydatid disease. The adult tapeworm ranges in length from 2 mm to 7 mm and has three proglottids ("segments") when intact — an immature proglottid, mature proglottid and a gravid proglottid. Like all cyclophyllideans, ''E. granulosus'' has four suckers on its scolex ("head"), and ''E. granulosus'' also has a rostellum with hooks. Several strains of ''E. granulosus'' have been identified, and all but two are noted to be infective in humans. The lifecycle of ''E. granulosus'' involves dogs and wild carnivores as a definitive host for the adult tapeworm.〔 Definitive hosts are where parasites reach maturity and reproduce. Wild or domesticated ungulates, such as sheep, serve as an intermediate host.〔 Transitions between life stages occur in intermediate hosts. The larval stage results in the formation of echinococcal cysts in intermediate hosts.〔 Echinococcal cysts are slow growing,〔 but can cause clinical symptoms in humans and be life-threatening. Cysts may not initially cause symptoms, in some cases for many years.〔 Symptoms developed depend on location of the cyst, but most occur in the liver, lungs, or both.〔 ''E. granulosus'' was first document in Alaska but is distributed world-wide. It is especially prevalent in parts of Eurasia, north and east Africa, Australia, and South America.〔 Communities that practice sheep farming experience the highest risk to humans,〔 but wild animals can also serve as an avenue for transmission. For example, dingoes serve as a definitive host before larvae infect sheep in the mainland of Australia.〔 Sled dogs may expose moose or reindeer to ''E. granulosus'' in parts of North America and Eurasia.〔 ==Transmission== ''E. granulosus'' requires two host types, a definitive host and an intermediate host. The definitive host of this parasite are dogs and the intermediate host are most commonly sheep, however, cattle, horses, pigs, goats, and camels are also potential intermediate hosts. Humans can also be an intermediate host for ''E. granulosus'', however this is uncommon and therefore humans are considered an aberrant intermediate host.〔 ''E. granulosus'' is ingested and attaches to the mucosa of the intestines in the definitive host and there the parasite will grow into the adult stages. Adult ''E. granulosus'' release eggs within the intestine which will be transported out of the body via feces.〔 When contaminated waste is excreted into the environment, intermediate host has the potential to contract the parasite by grazing in contaminated pasture, perpetuating the cycle.〔〔 ''E. granulosus'' is transmitted from the intermediate host (sheep) to the definitive host (dogs) by frequent feeding of offal, also referred to as “variety meat” or “organ meat”. Consuming offal containing ''E. granulosus'' can lead to infection; however, infection is dependent on many factors.〔 The frequency of offal feedings, the prevalence of the parasites within the offal, and the age of the intermediate host are factors that affect infection pressure within the definitive host.〔 The immunity of both the definitive and intermediate host plays a large role in the transmission of the parasite, as well as the contact rate between the intermediate and the definitive host (such as herding dogs and pasture animals being kept in close proximity where dogs can contaminate grazing areas with fecal matter).〔 The life expectancy of the parasite, coupled with the frequency of anthelminthic treatments, will also play a role in the rate of infection within a host. The temperature and humidity of the environment can affect the survival of ''E. granulosus''.〔 Once sheep are infected, the infection typically remains within the sheep for life. However in other hosts, such as dogs, treatment for annihilating the parasite is possible. However, the intermediate host is assumed to retain a greater life expectancy than the definitive host.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Echinococcus granulosus」の詳細全文を読む スポンサード リンク
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